feat(sprint-5.5): add block version snapshots with switching between versions

- Add BlockSnapshot Prisma model (html, css, version, changelog) + migration
- Add API endpoints: POST/GET /blocks/snapshots, GET /blocks/snapshots/:id
- BlockMetaBar: version dropdown, HTML capture on save, onSnapshotSelect prop
- "Сохранить версию" now captures innerHTML snapshot + CSS and stores in DB
- Selecting archived version shows stored HTML snapshot instead of live component
- Yellow banner "Архивная версия" with link to return to current
- Split all 8 block pages into Server Component (metadata) + Client Component
- Add data-block-capture attribute for snapshot capture targeting

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/api/prisma/migrations/20260324185731_add_block_snapshot/migration.sql

@@ -0,0 +1,18 @@
+-- CreateTable
+CREATE TABLE "BlockSnapshot" (
+    "id" TEXT NOT NULL,
+    "blockPath" TEXT NOT NULL,
+    "version" TEXT NOT NULL,
+    "changelog" JSONB NOT NULL DEFAULT '[]',
+    "html" TEXT NOT NULL,
+    "css" TEXT NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "BlockSnapshot_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "BlockSnapshot_blockPath_idx" ON "BlockSnapshot"("blockPath");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "BlockSnapshot_blockPath_version_key" ON "BlockSnapshot"("blockPath", "version");